An insight into the nature of the valve self-excited oscillation noise from a 1D simulation

In recent years, the number of electric vehicles has been increasing. Without the powertrain noise, the cabin of an electric vehicle becomes quiet. On the other hand, secondary noise such as air-conditioning (A/C) noise becomes more perceived with improvements of reduction techniques for other noise sources. Therefore, it becomes more important to reduce A/C noise. During one vehicle's development, an abnormal noise occurred. It was generated by a self-excited oscillation at the thermal expansion valve (TXV) and was one of A/C refrigerant flow-induced noise. To vanish it, a lot of bench and vehicle tests were conducted. The mechanism of the TXV self-excited oscillation noise was assumed from their test results. First, pressure pulsations with large amplitudes generated at the compressor suction enter from the opposite direction of flow in the TXV and oscillate the shaft and ball valve. Second, since the cross sectional area of the refrigerant channel varies, a large speed variation occurs a round inlet area of the ball valve. Finally, Continuous pressure pulsations in the liquid refrigerant also will be generated and will resonate with the liquid pipe. As a result, noise radiates from the liquid pipe to the air. To verify the assumed mechanism of generation, a simulation for the assessment was proposed. In this paper, a one dimensional (1D) refrigerant cycle model with the detailed TXV model was approached to develop. This detailed TXV model includes a fluid and structure interaction so that 1D simulation can virtually reproduce the occurrence of the TXV self-excited oscillation. To calculate pressure pulsations, flow channels in the model are discretized at each components and an explicit solver based on 1D Navier-Stokes equations is used. Moreover, Newton's equations of motion are used to calculate the balance of forces acting on the valve and then estimate transient lift amount. The simulation result shows the pressure oscillation seen when it occurs. Therefore, the assumed mechanism of self-excited oscillation noise is confirmed in this case. In addition, simulation was carried out with different conditions and insights into the nature can be obtained from results. For instance, as the condition that the pressure pulsation with large amplitude comes through the condenser in forward direction, amplitude is attenuated in calculation due to streaming in small pipe cross section. If lift amount of the valve is relatively small, it is shown a fluid and structure interaction tends to be strong. The degree of subcooling is large, calculation results show the high possibility to generate self-excited oscillation noise compared to the baseline. To test a hypothesis, experimental tests are conducted with different sized TXV and also in variation of charge amount of refrigerant. From measured pressure pulsations under some test conditions, good insights into this troublesome problem can be obtained.